1. Field of the Invention
The present invention relates to an iron beam irradiation device which draws out in a predetermined direction an ion beam from a plasma generated in a chamber through a grid to which a predetermined voltage is imparted, and applies the ion beam thus drawn out. More specifically, the present invention relates to a milling device which performs processing, for example, on the surface of a workpiece by using an ion beam drawn out, an ion implantation device which implants ions in a film, an ion beam film forming device which performs film formation, etc., and to an insulating member used as a spacer when fixing a grid in such devices.
2. Related Background Art
For example, a milling device using an ion beam uses a plasma generated in a chamber as an ion source, draws out ions from the ion source by using a plurality of grids to which a DC voltage is applied, and accelerates the ions in a predetermined direction, so as to perform milling by using the accelerated ions. Usually, the individual grids have holes for allowing passage of ions, and different DC voltages are applied thereto so as to optimize the ion energy drawn out and the distribution thereof. Here, the term milling refers to a process in which accelerated ions are caused to impinge upon the surface of a workpiece to generate a sputtering phenomenon to perform fine processing on the surface (see JP 2000-113849 A).
Between these grids, there is usually arranged an insulating spacer consisting of an insulator in order to prevent them from coming into contact with each other and to maintain a fixed distance between the grids. A specific example of the construction of a conventionally used insulating spacer will be described with reference to a drawing. FIG. 5 is a schematic sectional view of an insulating spacer for fixing three grids. An insulating spacer 20 for fixing grids 15, 16, and 17 is composed of a first insulating member 21, a second insulating member 25, an insulating cap 27, a mounting screw 29, and a nut 31.
The first insulating member 21 has a cylindrical large diameter portion 21a with a threaded outer periphery, a grid fixing portion 21b which is connected with the large diameter portion 21a and which has a tapered surface constituting a part of a cone whose outer diameter increases as it extends away from the large diameter portion 21a, and a small diameter portion 21c which is connected with the grid fixing portion 21b and whose outer diameter is smaller than that of the large diameter portion 21a. Further, the large diameter portion 21a, the grid fixing portion 21b, and the small diameter portion 21c are coaxial and formed in continuation, with a through-hole 21d being formed at the axial center thereof. Further, at the large diameter portion 21a side opening of the through-hole 21d, there is formed an enlarged-diameter portion 21e with an increased inner diameter. At the end of the small diameter portion 21c, there is formed a plane substantially perpendicular to the axial center, with and abutting the grid 17.
Formed in the grid 17 is a hole 17a through which the mounting screw 29 can be passed. When the mounting screw 29 is inserted into the through-hole 21d, the head portion of the mounting screw 29 is engaged with a step portion existing between the enlarged-diameter portion 21e and the through-hole 21d. The end portion of the screw portion of the mounting screw 29 in the inserted state protrudes from the hole 17a, and this protruding portion is threadedly engaged with the nut 31. Through the threaded-engagement operation, the grid 17 is held between the nut 31 and the end surface of the small diameter portion 21c, thus determining the positional relationship between the first insulating member 21 and the grid 17.
Formed in the grid 16 is a hole 16a whose diameter varies in correspondence with the tapered surface of the grid fixing portion 21b. The tapered surface of the hole 16a abuts the tapered surface of the grid fixing portion 21b. As a result, the positional relationship between the grid 16 and the first insulating member 21 is determined. The distance between the grid 16 and the grid 17 is determined by the outer periphery of the first insulating member 21 and the second, annular insulating member 25 arranged between these grids. The insulating cap 27 has a cylindrical configuration with one end closed and has at the open end an outwardly protruding flange portion 27a. A thread is formed inside the cylinder, and through threaded engagement of the thread in the outer periphery of the large diameter portion 21a with the above-mentioned thread, the insulating cap 27 is fixed with respect to the first insulating member 21.
The grid 15 is held and fixed between the second insulating member 25 and the flange portion 27a of the insulating cap. That is, by threadedly engaging the insulating cap 27 with the first insulating member 21 for fixation, the grid 16, the second insulating member 25, and the grid 15 are held between the grid fixing portion 21b and the flange portion 27a. As a result, their positional relationship with the first insulating member 21 is determined.
Here, when, for example, ion milling is performed by using an ion beam drawn out, sputtering is effected on the surface of a workpiece with this ion beam. Further, when drawing ions out of the chamber through the grids, not all of the ions accelerated by the grids reach the exterior of the chamber; a part of them may hit the grids, etc. to effect sputtering thereon. The substance thus struck out from the workpiece surface or the grids, etc. by this sputtering will be redeposited in the interior of the device. Regarding the first insulating member 21 shown here, what matters most is the redeposition of a substance on a surface 21f opposed to the grid 17 existing between the grid fixing portion 21b and the small diameter portion 21c, and on the outer peripheral surface of the small diameter portion 21c. FIG. 6 is an enlarged view of a region 22 where such redeposition occurs at high frequency.
When the redeposited substance is conductive, the insulation property of the above-mentioned insulating spacer, for example, deteriorates as the amount of redeposited substance increases, with the result that short-circuiting may finally occur between the grids. To prevent this, it is necessary to perform the operation of removing the redeposit film in a state in which the redeposition on the insulating spacer has progressed to some degree, or replace the insulating spacer by a new one. Thus, when, for example, processing is conducted on the metal film by milling operation, the insulation property abruptly deteriorates due to the redeposition of the metal, so that it is necessary to conduct the short-circuit preventing operation in a considerably short cycle. JP 2000-301353 A discloses a technology using an insulating spacer with a tapered sectional configuration.